Car overheats but engine does not seem hot

So I don't know much about cars so I really need good advice. I'm out here in the country, the next mechanic is 25 miles from here. My panel shows that the engine is overheating, but when I stop and open the hood, it's just regular warm. I was advised to turn on the heating to cool the engine (?), But the heating doesn't work either! There is coolant in the big reservoir. Can I drive to the mechanic with this? I cannot afford a tow. And what could this be? My dad (lives far away) said maybe just thermostat? I want to be prepared, because last time a mechanic kept my car for a long time, put a new transmission for it and that car never drove faster than 60 mph anymore. Please tell me what this could possibly be and what the average repair would be. I depend completely on this car.